Architects of The Air

Whether a nest lasts for decades or survives only a few weeks, it performs several functions. Nests are incubators for eggs, nurseries for young and hiding places for brooding parents, all vulnerable to predators and foul weather during nesting season.

Nest building is a complex activity and, although the ability is instinctive, birds get more skilled with practice. Some birds must make a thousand trips for materials to complete their nest. Others face the task of gouging out a cavity to use.

The drive to build nests is the result of a cascade of hormonal changes in birds that usually is brought on by lengthening daylight in the spring. Courtship, mating and brooding are all governed by hormones, though external factors also play a part.

The sound of bird song and the availability of food also have been shown to induce nesting. In addition, some birds build nests before they court or use nestbuilding as part of their courtship ritual. The mere sight of eggs induces some female birds to brood, while the site of brooding females can induce males of some species to sit on the clutch.

Nearly everyone can identify a robin's cup-shaped nest, fashioned of mud and grass. The reason they are so common is that robins usually build a new nest for each brood they raise. They've even been known to build several nests at once, sometimes side by side. Whether they do this because they are confused or to confuse predators isn't known, but both parents participate in this flurry of building.

Cardinals, finches, blue jays, mocking birds, crows and many other species also build some variety of the cup-shaped nest. Hummingbirds build the tiniest nest of all, little more than an inch across. It usually straddles a small, downward sloping branch and is easily overlooked. Female hummingbirds construct the nests and use spider webs and bits of lichen to disguise them. Because their nests are so tightly built, hummingbirds can return to them year after year.

The female red-eyed vireo builds a basketlike cup that hangs by its rim from the fork of a thin branch. She often uses grapevine bark in the construction. Male and female Baltimore orioles weave a pouch suspended from a drooping branch. The inside is lined with hair, fine grass and bits of string. This intricate pouch can take up to two weeks to construct.

Barn swallows and chimney swifts actually have benefited from human encroachment. Many

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